Small grain size, conformal aluminum interconnects and method for their formation

ABSTRACT

A first layer of titanium nitride (TiN) is formed on a semiconductor structure, such as an interconnect via. Then, a second layer of TiN is formed on the first layer of TiN. The first layer of TiN is amorphous. The second layer of TiN is polycrystalline, having a mixed grain orientation. Finally, an aluminum film is formed on the second layer of titanium nitride. Optionally, a titanium silicide layer is formed on the semiconductor structure prior to the step of forming the first layer of titanium nitride. Interconnects formed according to the invention have polycrystalline aluminum films with grain sizes of approximately less than 0.25 microns.

FIELD OF THE INVENTION

The present invention relates generally to semiconductor integratedcircuits and methods for their fabrication and, more particularly, toaluminum interconnects and methods for their formation.

BACKGROUND

Semiconductor integrated circuits (ICs) contain individual devices,which are typically operatively-coupled together using metal lines. Inmost applications, the metal lines are formed on a different level thanthe devices, separated by an intermediate dielectric, such as siliconoxide or borophosphosilicate glass (BPSG). The most commonly used metallines are aluminum. Interconnects are formed between individual devicesand the metal lines. A typical interconnect between metal layers iscomposed of a via (i.e. opening) formed in an intermediate dielectric.Similarly, an interconnect between a metal layer and silicon is composedof a contact (i.e. opening) formed in an intermediate dielectric over anactive device region. The via is filled with a metal, such as aluminumor tungsten. Aluminum has been preferred to date as an interconnectmetal. Aluminum exhibits relatively low resistivity as compared totungsten and, furthermore, is highly compatible with silicon oxide,which is often used as the insulative material surrounding a via.Furthermore, when metal lines are used, which are composed of aluminum,compatibility between the metal lines and the aluminum interconnectmaterials is optimized.

Interconnects often further contain a diffusion barrier layer sandwichedbetween the metal and the active device region at the bottom of the via.Such layers prevent intermixing of the metal and material from theactive device region, such as silicon, which extends the life of thedevice. Passive titanium nitride diffusion barrier layers are the mostcommon diffusion barrier layers. Such layers are typically formed over arefractory metal silicide layer. Titanium silicide is the most commonlyused refractory metal silicide due to its relatively low resistivity.The use of titanium silicide between titanium nitride and the activedevice region is preferred due to its intermediate crystallographiccharacteristics between those of silicon and titanium nitride. Theintermediate crystallographic characteristics prevent increasedresistivity resulting from a contact solely between silicon and titaniumnitride, whose crystallographic characteristics are very different.

Ideally, interconnects exhibit zero impedance to current flow, asexhibited in an ohmic contact (i.e. those which exhibit linear currentvs. voltage characteristics), to provide optimum electrical performance.However, interconnects are not ideal and typically exhibit near linearcharacteristics at best.

One significant concern in depositing metal into a via is obtainingadequate step coverage of the via, particularly obtaining adequate stepcoverage is difficult when the vias have high aspect ratios (i.e. alarge ratio of height to width of the via), as seen more often as ICdensities increase. To mitigate this problem, chemical vapor deposition(CVD) is used to deposit the metal instead of physical vapor deposition(PVD). CVD is more apt to adequately fill high-aspect ratio contactholes than PVD. However, to date, CVD aluminum exhibits rough,nonconformal layers on complex topographies, prior to surfacemodification. This is undesirable because voids often develop within avia, due to the roughness of the CVD aluminum. Such voids severelyincrease the resistivity of a contact and degrade device performance bynot providing uniform connection across an interconnect.

There is a need for an interconnect structure that effectively utilizesaluminum instead of tungsten. There is a further need for a method forforming a smooth, conformal aluminum layer within an interconnectstructure. A method for using aluminum in sub-0.25 micron contact holesis needed in order to optimize future device performance.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for fabricating aninterconnect supported by a semiconductor structure. A first layer oftitanium nitride is formed on the semiconductor structure. Then, asecond layer of titanium nitride is formed on the first layer oftitanium nitride. Finally, an aluminum film is formed on the secondlayer of titanium nitride. A titanium silicide layer is optionallyformed on the semiconductor structure prior to the step of forming thefirst layer of titanium nitride.

In particular, according to one aspect of the method of the presentinvention, a first titanium nitride layer is formed on an active deviceregion to act as a barrier layer, protecting the integrity of thecontact. To accomplish this task, an amorphous titanium nitride layer isformed by reacting a titanium-containing precursor in the presence ofnitrogen. Then, a second titanium nitride layer is formed on the firsttitanium nitride layer. The second titanium layer has a polycrystallineorientation (having a mixture of grains orientated in the <111> and<200> directions), which allows diffusion between the active deviceregion and the interconnect metal. By forming a polycrystalline layerover the amorphous barrier layer, subsequent formation of small grainsize chemical vapor deposition (CVD) aluminum is possible. The secondtitanium nitride layer is formed by reacting a titanium-containingprecursor in the presence of at least ammonia (NH₃) or nitrogentrifluoride (NF₃). The titanium-containing precursors for forming thefirst and second layers of TiN are selected from the group consistingof: titanium tetrachloride, tetrakisdimethylamide titanium, andtrimethylethylenediamino titanium. Finally, CVD aluminum is formed onthe second titanium nitride layer to complete the interconnect. To formthe CVD aluminum, an aluminum-containing precursor is used. Thealuminum-containing precursor is selected from the group consisting of:trimethylaluminum (TMA), dimethylaluminum hydride (DMAH),triisobutylaluminum (TIBA), triethylaluminum (TEA), diethylaluminumhydride (DEAH), monomethylaluminum hydride (MMAH), dimethylethylalane(DMEHA1), and dimethylethylamide (DMEHA2).

In particular, according to another aspect of the method of the presentinvention, a titanium silicide layer is formed between the firsttitanium nitride layer and the underlying active device region tofurther improve the ohmic characteristics of the contact. Titaniumsilicide has an intermediate crystallographic structure between that ofsilicon in the active device region and titanium nitride. Thus,electronic carriers are able to diffuse easily through the interconnectstructure.

The crystal structure and grain size of the underlayers are controlledto promote subsequent formation of a small grain size, conformalaluminum film in an interconnect. The aluminum film formed according tothe method of the invention has a polycrystalline orientation, withgrain sizes of less than approximately 0.25 microns. Thus, interconnectsare formed with aluminum instead of tungsten, providing a lowerresistivity contact to adjacent active device regions. Furthermore, suchaluminum films are more conformal due to their smaller grain sizes. Viasare thus able to be filled with such aluminum to provide an interconnectstructure substantially free of unwanted voids.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional representation of an aluminum interconnectformed in accordance with the method of the invention.

FIG. 2 is a cross-sectional representation of an embodiment wherein atitanium silicide layer is formed between an active device region andthe interconnect.

FIG. 3 is a cross-sectional representation of an embodiment wherein adamascene aluminum interconnect is formed using the method of theinvention.

FIG. 4 is a cross-sectional representation of an embodiment wherein adamascene aluminum interconnect is formed over a titanium silicide layerusing the method of the invention.

FIG. 5 is a cross-sectional representation of a multi-level interconnectstructure.

DESCRIPTION OF THE INVENTION

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings which form a part hereof,and in which is illustrated by way of specific embodiments in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and that structural or logical changes maybe made without departing from the scope of the present invention. Forexample, the terms wafer and substrate used in the following descriptioninclude any semiconductor-based structure. Wafer and substrate are usedinterchangeably to refer to supporting semiconductor structures 20during processing, as illustrated in FIGS. 1 to 4. Both are to beunderstood as including silicon-on-sapphire (SOS) technology,silicon-on-insulator (SOI) technology, doped and undoped semiconductors,epitaxial layers of a silicon supported by a base semiconductor, as wellas other semiconductor structures well known to one skilled in the art.Furthermore, when reference is made to a wafer or substrate in thefollowing description, previous process steps may have been utilized toform regions/junctions in the base semiconductor structure 20. Thefollowing detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present invention is defined by theappended claims.

The following described embodiments of the present invention aredescribed as applied to forming a small grain size, conformal aluminumfilm in a via or contact, referred to herein as an interconnect via, inan integrated circuit (IC). However, the claims are not meant to be readas limited to forming such an aluminum film only in IC interconnectvias. The invention is particularly useful for forming aluminum films insub-0.25 micron interconnect vias (contact holes) and interconnect viashaving high-aspect ratios (i.e., ratio of height to diameter of aninterconnect via), such as those greater than approximately 5:1. Inparticular, source/drain active regions of metaloxide-semiconductor(MOS) transistors have a particularly high aspect ratio of approximately8:1. Previously, conformal aluminum films were difficult to form in suchstructures. By using the method of the invention, lower resistivityaluminum is able to be used in interconnect vias, instead of tungsten,providing better IC performance. This is particularly critical insub-0.25 micron technology applications.

Interconnect structures illustrated in FIGS. 1 to 4 are formed accordingto the method of the invention. In the following example, aninterconnect is etched in an isolating material 22, such as siliconoxide or borophosphosilicate glass (BPSG) to define a semiconductorstructure having a bottom surface, a top surface, and sidewalls of aninterconnect via. The bottom surface of the interconnect via is anactive region 24 of an IC, such as a source/drain region in atransistor. The top surface and sidewalls of the interconnect viacomprise the isolating material 22 in which the interconnect via isetched.

The semiconductor structure is then placed in a chemical vapordeposition (CVD) reaction chamber. A titanium-containing organometallicprecursor, such as tetrakisdimethylamido titanium (TDMAT) ortrimethylethylenediamino titanium (TMEDT), or anothertitanium-containing precursor gas, such as titanium tetrachloride isreacted in the CVD chamber. The main criteria for selecting atitanium-containing precursor gas is to assure that the precursor gas iscapable of reacting in a CVD chamber to form a titanium nitride (TiN)film.

A first layer of TiN 26 is formed on the semiconductor structure,extending into the interconnect via, by reacting the titanium-containingprecursor with nitrogen gas. The titanium-containing precursor is flowedat approximately 10 to 100 sccm. Nitrogen is flowed at approximately 10to 1,000 sccm. Pressure in the CVD chamber is maintained atapproximately 0.1 to 760 Torr. Temperature in the CVD chamber ismaintained at approximately below the respective decompositiontemperature for the precursor, but at a temperature that permits theprecursor to be sufficiently capable of being volatilized in the CVDchamber. This method is applicable to any suitable type of CVD chamber,including cold-wall and hot-wall chambers. Consequently, temperature isalso adjusted according to the type of CVD chamber used. These pressuresand temperatures do not, however, appear to be highly critical to thedeposition of the TiN layer 26.

By using the method described for forming the first layer of TiN 26, theresulting TiN film 26 is amorphous, preventing unwanted diffusionbetween the active device region 24 and the subsequently formed aluminum30. The thickness of the first layer of TiN 26 is proportionate to thedimensions of the interconnect via. For example, for a 0.25 microninterconnect via, the thickness is approximately 100 to 200 angstroms.

A second layer of TiN 28 is then formed on the first layer of TiN 26 byreacting a titanium-containing precursor as described above with atleast one of the following gases: ammonia and nitrogen trifluoride. Thetitanium-containing precursor is flowed at approximately 10 to 100 sccm.Ammonia or nitrogen trifluoride is flowed at approximately 10 to 1,000sccm. Optionally, other gases, such as inert carrier gases or nitrogen,can be used in the CVD chamber during reaction to form the second layerof TiN 28. Such optional gases are flowed at approximately 10 to 1,000sccm. Pressure in the CVD chamber is maintained at approximately 0.1 to760 Torr. Temperature in the CVD chamber is maintained at approximatelybelow the respective decomposition temperature for the precursor, but ata temperature such that the precursor is sufficiently capable of beingvolatilized in the CVD chamber. This method is applicable using anysuitable type of CVD chamber, including cold-wall and hot-wall chambers.Thus, temperature is also adjusted according to the type of CVD chamberused. Again, as in the deposition of TiN layer 26, pressures andtemperatures do not appear to be highly critical to the deposition ofthe TiN layer 28.

By using the method described for forming the second layer of TiN 28,the resulting TiN film 28 has a polycrystalline orientation. Inparticular, the grain orientation of the second layer of TiN 28comprises a mixture 1:1 of <111> and <200> orientated grains. It ispreferred that this layer of TiN have a polycrystalline orientation, sothat the subsequently formed aluminum 30 will have a small grain sizeand conformal shape due to the abundance of grain nucleation sites atboundaries between adjacent grains in the second TiN layer 28. Such aplurality of nucleation sites enables small grains to form in thesubsequently formed aluminum 30. When there are many nucleation sitespresent, grains will nucleate and impinge on each other, preventing anyof the grains from easily growing too big. The thickness of the secondlayer of TiN 28 is proportionate to the dimensions of the interconnectvia. For example, for a 0.25 micron interconnect via, the thickness isapproximately 100 to 200 angstroms.

An aluminum-containing organometallic precursor, such astrimethylaluminum (TMA), dimethylaluminum hydride (DMAH),triisobutylaluminum (TIBA), triethylaluminum (TEA), diethylaluminumhydride (DEAH), monomethylaluminum hydride (MMAH),dimethylethylaminealane (DMEAA), or dimethylethylamide (DMEHA) isreacted in the CVD chamber in the presence of a carrier gas, such asinert gases to form the aluminum interconnect 30. The thickness of thealuminum 30 is proportionate to the dimensions of the interconnect via.For example, for a 0.25 micron interconnect via, the thickness of thealuminum 30 within the interconnect via is approximately 2,000 to 3,000angstroms. However, more aluminum 30 is also deposited outside of theinterconnect via. For example, for a 0.25 micron interconnect via,approximately 1,000 to 2,000 angstroms of aluminum is also depositedabove the top surface of the interconnect via. Then, excess TiN 26, 28and aluminum 30 is removed to form an interconnect structure as wellknown to one skilled in the art and illustrated in FIG. 1. The presenceof multiple, small grains in the aluminum 30 enable the aluminum 30 toconform more easily to surfaces having complex topographies, such ashigh-aspect ratio interconnect vias. Improved conformality in aluminuminterconnect 30 prevents formation of voids within the interconnect.

In another embodiment, a refractory metal silicide layer 32 is formed onthe bottom surface of the interconnect via prior to formation of thefirst or second TiN layers 26 and 28 and the aluminum 30, as illustratedin FIG. 2. The presence of a refractory metal silicide layer 32 betweenthe active region 24 and the TiN diffusion barrier layer 26 decreasesresistivity of the contact by providing a layer of material 32 having anintermediate grain structure between that of silicon in the activeregion 24 and titanium in the TiN layer 26.

To form the refractory metal silicide layer 32, a refractory metal 34,such as titanium, is deposited over the semiconductor structure,extending into the interconnect via to a thickness proportional to thedimensions of the interconnect via. For example, for a 0.25 microninterconnect via, the thickness is approximately 100 to 200 angstroms.The structure is then annealed to form the refractory metal silicide 32,such as titanium silicide, on those surfaces containing silicon, thatsupport the titanium layer 34. Typically, the active region 24 overwhich an interconnect is formed contains silicon. Therefore, titaniumsilicide 32 forms only on the bottom surface of a interconnect viaoverlying an active region 24. The remaining titanium 34 remainsunreacted on the semiconductor structure.

In yet another embodiment, the final interconnect structures illustratedin FIGS. 1 and 2 are polished so that the top surface of the finalinterconnect is coplanar with the top surface of the interconnect via,as illustrated in FIGS. 3 and 4. Any type of polishing technique is usedto form the damascene structures illustrated in FIGS. 3 and 4.Typically, chemical mechanical polishing is used to polish thestructure, as well known to one skilled in the art. FIG. 4 illustratesone embodiment of the invention, in which a refractory metal silicidelayer 32 is also formed in the damascene interconnect. The resultingdamascene structures utilize the aluminum 30 within the interconnect viaas both an interconnect metal and metal line structures, which connectadjacent devices in an IC. By using a damascene structure, the need forsubsequently forming metal lines on the interconnect structure forconnecting adjacent active devices is eliminated.

FIG. 5 illustrates a second interconnect connection formed between thefirst metal layer 30 and a second metal layer 40. These layers can bealuminum which is connected as described above using an interconnect viaextending through an intermediate layer(s) of oxide 42. A layer of TiN44 is provided as described above for layer 28. Thus, multiple levels ofinterconnects (both contacts and vias) can be formed using the describedmethods.

The invention was described above with reference to forming lowresistivity interconnects in an IC. However, the method utilized forforming the small grain size, conformal aluminum films 30 is applicableto forming aluminum films anywhere as desired over a supportingsemiconductor structure. Resulting aluminum films 30 have a lowerresistivity than other types of metal films currently used, such astungsten, and have a conformal shape due to the small grain size in thealuminum 30. Thus, such films 30 are able to be easily formed overcomplex topographies. In particular, when filling an interconnect viawith the aluminum 30, voids are prevented from forming within theinterconnect via. Unwanted voids degrade device performance, oftencausing shorts in the circuit.

It is to be understood that the above description is intended to beillustrative, and not restrictive. Many other embodiments will beapparent to those of skill in the art upon reviewing the abovedescription. The scope of the invention should, therefore, be determinedwith reference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

What is claimed is:
 1. A method of fabricating an interconnect supportedby a semiconductor structure, comprising: forming a first layer oftitanium nitride on the semiconductor structure; forming a second layerof titanium nitride on the first layer of titanium nitride; and formingan aluminum film on the second layer of titanium nitride wherein thesecond layer of titanium nitride comprises a polycrystalline orientationthat comprises a mixture of 1:1 of <111> and <200> oriented grains thatare effective for forming an aluminum film of small grain size.
 2. Themethod of claim 1, further comprising forming a titanium silicide layeron the semiconductor structure prior to the step of forming the firstlayer of titanium nitride.
 3. The method of claim 1, wherein forming thefirst layer of titanium nitride comprises: flowing a titanium-containingprecursor in a chemical vapor deposition reaction chamber; and flowingnitrogen in the chemical vapor deposition chamber simultaneously withflowing the titanium-containing precursor.
 4. The method of claim 3,wherein the titanium-containing precursor is selected from the groupconsisting of titanium tetrachloride, tetrakisdimethylamido titanium,and trimethylethylenediamino titanium.
 5. The method of claim 1, whereinforming the second layer of titanium nitride comprises: flowing atitanium-containing precursor in a chemical vapor deposition reactionchamber; and flowing a gas selected from the group consisting of ammoniaand nitrogen trifluoride in the chemical vapor deposition reactionchamber simultaneously with the step of flowing the titanium-containingprecursor.
 6. The method of claim 5, further comprising flowing nitrogenin the chemical vapor deposition reaction chamber simultaneously withflowing the titanium-containing precursor.
 7. The method of claim 5,wherein the titanium-containing precursor is selected from the groupconsisting of titanium tetrachloride, tetrakisdimethylamido titanium,and trimethylethylenediamino titanium.
 8. The method of claim 1, whereinforming the aluminum film comprises flowing an aluminum-containingprecursor in a chemical vapor deposition chamber.
 9. The method of claim8, wherein the aluminum-containing precursor is selected from the groupconsisting of trimethylaluminum (TMA), dimethylaluminum hydride (DMAH),triisobutylaluminum (TIBA), triethylaluminum (TEA), diethylaluminumhydride (DEAH), monomethylaluminum hydride (MMAH), dimethylethylalane(DMEHA1), and dimethylethylamide (DMEHA2).